1. Field of the Invention
The present invention relates to a magneto-optical recording medium used for recording or reproducing information, a method for producing the same, and an apparatus for producing the same.
2. Description of the Related Art
As a repeatedly rewritable recording medium having a high density, a magneto-optical recording medium and a recording/reproducing apparatus for recording a minute domain onto a magnetic thin film with thermal energy of laser light, and reproducing a signal using a magneto-optical effect are being developed actively. In such a magneto-optical recording medium, when the diameter and interval of recording bits (domains for recording) become smaller with respect to the diameter of a light beam focused onto the medium, reproduction characteristics are degraded. This is caused as follows: an adjacent recording bit enters the diameter of a light beam focused onto an intended recording bit, which makes it difficult to reproduce information from individual recording bits separately.
In order to solve the above-mentioned problem, attempts have been made to enhance a recording density by modifying the configuration of a recording medium and a reproducing method. For example, a super-resolution system, a domain wall displacement detection (DWDD) reproducing system using the displacement of a domain wall, and the like have been proposed. Herein, a DWDD reproducing system disclosed in JP6(1997)-290496 A will be described with reference to FIG. 9.
In a magneto-optical recording medium shown in FIG. 9, a reproducing layer (domain wall displacement layer) 91, an intermediate layer (switching layer) 92, and a recording layer 93 that constitute magnetic layers 90 are exchange-coupled to each other, and a minute recording domain of the recording layer 93 is enlarged in the reproducing layer 91, whereby an amplitude of a reproducing signal is increased, making it possible to conduct high-density recording. Arrows represent the sublattice magnetization directions of transition metal in each layer. In each layer, a domain wall 94 is formed between domains in which magnetization directions are opposite to each other. A region 95 of the intermediate layer 92 reaches a temperature equal to or higher than a Curie temperature due to the irradiation with laser light for reproduction, whereby a magnetic order is lost.
The conditions desired for the above-mentioned magneto-optical recording medium are summarized by the following four points:
(1) The magneto-optical recording medium has the recording layer 93 that holds minute domains stably in a range from a room temperature to a reproducing temperature.
(2) Even when the magneto-optical recording medium is heated to the vicinity of a Curie temperature of the intermediate layer 92, the reproducing layer 91, the intermediate layer 92, and the recording layer 93 are exchange-coupled to each other.
(3) When the intermediate layer 92 reaches a temperature exceeding its Curie temperature so as to lose its magnetic order, exchange coupling between the recording layer 93 and the reproducing layer 91 is cut off.
(4) The domain wall coercive force of the reproducing layer 91 is small, and a domain wall energy gradient is caused by a temperature gradient. Therefore, in a region of the reproducing layer 91 where exchange coupling is cut off by the intermediate layer 92, the domain wall 94 is displaced from a position transferred from a domain of the recording layer 93. As a result, the magnetization in this region is aligned in the same direction, and an interval (recording mark length) between the magnetic walls 94 of the recording layer 93 is enlarged.
In FIG. 9, when the magneto-optical recording medium is moved (rotated in the case of a disk) in the right direction on the drawing surface while laser light is radiated thereto, due to the high linear velocity of the medium, the position at which a film temperature becomes maximum is placed on the backward side from the center of a beam spot in a traveling direction (left direction on the drawing surface) thereof. A domain wall energy density "sgr"1 in the reproducing layer 91 generally decreases with an increase in temperature to become 0 at a temperature equal to or higher than a Curie temperature. Therefore, in the presence of a temperature gradient, the domain wall energy density "sgr"1 is decreased toward a higher temperature side.
Herein, a force F1 represented by the following expression acts on a domain wall present at a position xe2x80x9cxxe2x80x9d in a medium movement direction (circumferential direction of a disk).
xe2x80x83F1∞xe2x88x92d"sgr"1/dx 
The force F1 acts so as to displace a domain wall in a direction of lower domain wall energy. In the reproducing layer 91, a domain wall coercive force is smaller and a domain wall mobility is larger compared with those of the other magnetic layers. Therefore, when exchange coupling from the intermediate layer 92 is cut off, a domain wall is displaced very rapidly in a direction of lower domain wall energy due to the force F1.
Referring to FIG. 9, in a region of the medium before being irradiated with laser light (e.g., a region at a room temperature), three magnetic layers are exchange-coupled to each other, and domains recorded in the recording layer 93 are transferred to the reproducing layer 91. In this state, the domain walls 94 are present between domains having magnetization directions opposite to each other in each layer. In the region 95 that reaches a temperature equal to or higher than the Curie temperature of the intermediate layer 92 due to the irradiation with laser light, magnetization of the intermediate layer 92 is lost, and the exchange coupling between the reproducing layer 91 and the recording layer 93 is cut off. Therefore, a force for holding a domain wall is lost in the reproducing layer 91, and a domain wall is displaced to a higher temperature side due to the force F1 applied to the domain wall. At this time, a domain wall displacement speed is sufficiently higher than that of the medium movement speed. Thus, a domain larger than a domain stored in the recording layer 93 is transferred to the reproducing layer 91.
In a magneto-optical recording medium using the DWDD reproducing system, for the purpose of displacing a domain wall easily, the following is proposed: guide grooves having a rectangular cross-section are formed on a substrate so that domain walls are not generated on the side of the recording tracks, whereby the respective tracks are separated by the grooves. However, even if guide grooves having a rectangular cross-section are formed, films actually are accumulated to some degree in stepped portions, and magnetic layers are connected to each other. As a result, magnetic separation cannot be conducted completely, which inhibits the displacement of a domain wall.
The magneto-optical recording medium of the present invention includes a substrate and a multi-layer film formed on the substrate, the multi-layer film including a first dielectric layer, a domain wall displacement layer, a switching layer, a recording layer, and a second dielectric layer in this order from the substrate side, a Curie temperature of the switching layer being lower than those of the domain wall displacement layer and the recording layer, a domain wall in the domain wall displacement layer being displaced to a higher temperature side in a region that reaches a temperature equal to or higher than a Curie temperature of the switching layer due to irradiation with a light beam for reproduction. In the magneto-optical recording medium of the present invention, the magnetic anisotropy of at least one layer selected from the group consisting of the domain wall displacement layer and the recording layer formed between recording tracks is made lower than that of said layers on the recording tracks, and magnetization of at least one magnetic layer selected from the group consisting of the domain wall displacement layer, the switching layer, and the recording layer is aligned in a predetermined direction in a region that is a half or more of a track width in a track width direction in at least a part of the recording tracks.
In the above-mentioned magneto-optical recording medium, it is preferable that magnetization of at least the recording layer is aligned in the predetermined direction.
Furthermore, it is preferable that magnetization is aligned perpendicularly to a film surface on the recording tracks, and that magnetization is aligned in parallel with a film surface (in a film surface direction) between the recording tracks. When magnetization is aligned in a film surface direction between the recording tracks, a domain wall displacement speed on the recording tracks can be increased. It further is preferable that magnetization is aligned in an extension direction of the recording tracks therebetween. In such an alignment, a leakage magnetic field in a radial direction can be decreased, and a shielding effect between the recording tracks can be increased.
In the above-mentioned magneto-optical recording medium, magnetization may be aligned in the predetermined direction on all the recording tracks, and alignment directions of magnetization on the recording tracks may be varied depending upon the recording tracks. In the latter case, it is preferable that the alignment directions are reversed at each track (at each recording track), or that the alignment directions are reversed at every other recording track. This is because an influence of a leakage magnetic field further is reduced.
Although not particularly limited, when the present invention is applied to a magneto-optical recording medium in which a pit and a groove are embossed on a substrate, and a track pitch of the recording tracks is 0.9 xcexcm or less, satisfactory results are obtained. Furthermore, the present invention is suitable for a magneto-optical recording medium in which the recording track is composed of segments containing a pit region and a data region, wobble pits for sampling servo are formed in the pit region, grooves and lands are formed in the data region, and the grooves are used as recording tracks.
In order to achieve the above-mentioned object, a method of the present invention for producing a magneto-optical recording medium having the above-mentioned configuration includes: irradiating a light beam between the recording tracks of the magneto-optical recording medium, thereby making the magnetic anisotropy of at least one layer selected from the group consisting of the domain wall displacement layer and the recording layer formed between the recording tracks lower than that of said layers on the recording tracks; and applying a bias magnetic field while irradiating the light beam at least between the recording tracks.
According to the production method of the present invention, because of the application of a bias magnetic field, the perpendicular magnetic anisotropy between recording tracks can be reduced effectively, and a magnetic interaction with respect to the recording track can be decreased. Furthermore, the application of a bias magnetic field also can be used for initializing recording tracks. More specifically, magnetization of the recording layer may be aligned in a predetermined direction in a width direction in at least a part of the recording tracks by applying a bias magnetic field.
In the above-mentioned production method, specifically, a light beam focused to be smaller than a light beam for reproduction may be radiated between the recording tracks.
In the above-mentioned production method, it is preferable that a bias magnetic field is applied perpendicularly to a film surface. In this case, an application direction of a bias magnetic field may be the same between all the recording tracks. However, when the application direction is reversed at each recording track or at every other recording track, an effect of a leakage magnetic field can be reduced. In the above-mentioned production method, a bias magnetic field may be applied in an extension direction of the recording tracks in parallel with a film surface.
In the above-mentioned production method, a bias magnetic field to be applied may be 150 Oe or more. Furthermore, a light beam focused by an objective lens with a numerical aperture of 0.65 or more may be radiated between the recording tracks.
In order to achieve the above-mentioned object, according to the present invention, an apparatus for producing a magneto-optical recording medium having the above-mentioned configuration is provided. The production apparatus includes a light beam irradiation apparatus for irradiating a light beam between recording tracks of the magneto-optical recording medium; a magnetic field application apparatus for applying a bias magnetic field at least between the recording tracks while irradiating the light beam; and a magnetic field control apparatus for changing a direction of the bias magnetic field.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.